The invention relates to bowls of carbon material, in particular of carbon/carbon (C/C) composite material, used for receiving a crucible in a metallurgical installation.
A particular field of application of the invention is that of bowls for receiving crucibles for drawing silicon single crystals.
A well-known process of producing a silicon single crystal, in particular for the purpose of fabricating semiconductors, consists in melting silicon in a receptacle, in putting the bath of liquid silicon into contact with a seed crystal having the desired crystal configuration, thereby initiating solidification of the silicon with this crystal configuration, and in mechanically drawing the resulting single crystal out from the crucible. That process is known as the Czochralski process or the xe2x80x9cCzxe2x80x9d process.
The receptacle containing the molten silicon is frequently a crucible made of silica or of quartz (SiO2) placed in a bowl, which bowl is sometimes referred to as a xe2x80x9csusceptorxe2x80x9d. Heating can be provided by radiation from a cylindrical graphite body, e.g. a body heated by the Joule effect and surrounding the bowl.
Traditionally, the bowls used in Cz processes have been made of graphite. The graphite pieces used as bowls are fragile. They are often built up using a plurality of parts and therefore cannot retain molten silicon in the event of the crucible breaking or leaking. This safety problem becomes critical when the size of the silicon crystals drawn becomes large, since there is a corresponding increase in the mass of liquid silicon. Furthermore, graphite bowls generally have a lifetime that is short and thickness that is large, and are thus massive and bulky.
In order to avoid those drawbacks, proposals have been made to make bowls out of C/C composite material comprising carbon fiber reinforcement densified with a carbon matrix. Such a material has much better mechanical strength than graphite and makes it possible to envisage producing bowls of large diameter, for example of diameter reaching or even exceeding 850 millimeters (mm) so as to be able to satisfy the demand for silicon single crystals of large section. In addition, the wall thickness of such bowls can be reduced compared with the wall thickness of graphite bowls, thereby facilitating the transmission of heat flux to the crucible and reducing mass and bulk.
The fabrication of pieces made out of C/C composite material is relatively expensive, particularly when such pieces are of a shape that cannot be developed and that is hollow, as applies to bowls. It is important to ensure that bowls have a service life that is as long as possible and therefore to protect the bowls against corrosion caused by chemical reaction between the material constituting the crucible and the carbon of the bowl.
With crucibles made of silica, the problem of corrosion is known. It involves SiO being given off and silicon carbide (SiC) being formed which, in turn, can react with the silica of the crucible, thereby giving rise to progressive consumption of the C/C composite and a decrease in the wall thickness of the bowl which puts a limit on its service lifetime.
To resolve this problem, proposals have been made to make C/C composite material bowls that are double-walled: an outer wall of C/C composite material having high mechanical strength and an inner wall which is at least partially of C/C composite material and which constitutes a replaceable, consumable lining. Such a solution, as described in particular in Japanese patent applications published under the numbers JP 9-263 482 and JP 10-25 185, is not satisfactory from the cost point of view. Making an inner wall or at least a portion of an inner wall out of C/C material remains lengthy and expensive, even when lower mechanical strength is required.
It should be observed that placing a graphite or a C/C composite material protective structure between the crucible and the bowl is also known for graphite bowls built up from a plurality of sections, as described in documents JP 10-158089 and DE 40 07 053.
Proposals have also been made, in particular in the Japanese patent application published under the No. JP 10-059795, to provide the inside surface of a C/C composite bowl with a coating of high purity pyrolytic carbon. The coating is made by chemical vapor deposition (CVD) which adds a relatively lengthy and expensive operation to the process of fabricating the bowl. In addition, such a coating is not exempt from being chemically corroded by the material of the crucible and in the long run will expose the C/C composite material to corrosion and possibly require a new operation of chemical vapor deposition to be performed.
There therefore exists a need to solve the problem of protecting the C/C composite material of the bowl against being chemically corroded by the material of the crucible, in a manner that is simple and inexpensive.
The Applicant has also found another problem associated with the interaction between the crucible and the bowl being not only chemical, but also physical.
In operation, the silica crucible softens and fits closely to the inside face of the bowl containing it. On cooling, a heat-shrink effect occurs where the bowl tightens against the crucible because of differences in the way the dimensions of the crucible and the bowl materials vary. This causes stresses to be applied to the bowl.
With graphite bowls, this problem of stresses is solved by the bowls being made up of a plurality of segments. However, with C/C composite bowls, these stresses can limit lifetime or can give rise to residual deformation.
In addition, heat shrinking of the bowl onto the crucible makes it difficult to remove the crucible at the end of each crystal-drawing cycle.
Furthermore, the effects of the chemical and physical phenomena amplify each other mutually.
An object of the invention is to propose a solution to the above-specified problems that is simple and of low cost, making it possible to reduce considerably the extent to which the crucible material corrodes bowls made of carbon material, in particular C/C composite material, and making it possible to limit the mechanical stresses that are generated in bowls, thereby lengthening the service lifetime of bowls.
This object is achieved by a method of mounting a crucible in a carbon material bowl in which method, prior to housing the crucible in the bowl, a consumable buffer ply is applied over at least a portion of the inside face of the bowl, the consumable ply being constituted essentially by a non-rigid carbon fiber fabric.
By way of example, a non-rigid carbon fiber fabric is a woven cloth, a knit, a multidirectional sheet, or a thin felt, with such a fabric having flexibility that enables it to adapt to the shape of at least a concave portion of the inside face of the bowl. Such a fabric also has macropores and offers limited contact area with the solid surface against which it is applied.
The invention is remarkable in that in spite of its small thickness and its porosity, such a buffer ply provides protection which is effective, as described in greater detail below. In addition, such a ply, even though it needs to be replaced periodically, and as a result constitutes a consumable, nevertheless provides protection at relatively low cost.
In addition, such a buffer ply, because it has the ability to deform in its thickness and because it is not bonded to the material of the bowl, limits the extent to which stresses are generated in the bowl as a result of differential variations in dimensions between the bowl and the crucible, and makes it easier to withdraw the crucible at the end of a crystal-drawing cycle.
The buffer ply can be made of a deformable fabric capable of fitting to the shape of at least a portion of the inside face of the bowl merely by deformation within the fabric, and without forming creases.
It is also possible to use a buffer ply made of a fabric that is precut so as to enable it to fit closely to the shape of at least a portion of the inside face of the bowl.
According to a feature of the method, prior to putting the buffer ply into place in the bowl, a thin coating of pyrolytic carbon can be formed on the fibers of the ply. Such a thin coating can be formed so as to xe2x80x9cfixxe2x80x9d the fibers and prevent them from becoming separated while the bowl is in use. The thin coating of pyrolytic carbon could also be made on the buffer ply while it is held in a desired shape, so as to xe2x80x9cpreformxe2x80x9d the buffer ply into a shape that is at least close to the desired shape, while not causing the ply to become rigid.
In practice, the thickness of the coating of pyrolytic carbon on the fibers should not exceed 4 microns (xcexcm), and is preferably no greater than 2 xcexcm. The thin coating of pyrolytic carbon can also avoid impurities diffusing from the fibers to the crucible.
According to yet another feature of the method, purification heat treatment is performed on the buffer ply.
The invention also seeks to provide a metallurgical installation, in particular for drawing silicon single crystals, the installation comprising a support bowl of C/C composite material, a crucible received in the bowl, and bowl protection means interposed between the bowl and the crucible, in which installation the protection means comprise a buffer ply as defined above.